This project deals with a process of translational control that affects several major mRNA species in a variety of mouse cells, as well as rat and human cells. A considerable portion of the mRNA molecules of these species are maintained in an untranslated state in the cytoplasm, presumably as a means to control the synthesis of the corresponding proteins. The significance of this process will be investigated by determining the nature and function of these proteins and identifying the physiological states associated with the activation and repression of the mRNAs. The amino acid sequences of the polypeptides will be deduced through cDNA sequencing, synthetic oligopeptides derived from these sequences will be used to generate antibodies, and the subcellular localization of the polypeptides will be determined by immunoprecipitation. Rates of synthesis of the polypeptides will be determined in cells in various physiological states, in order to identify conditions that may lead to activation or repression of the mRNAs. Cytoplasmic factors that may act as signals for activation or repression will be sought using a cell-free translation system. The structural basis for the repressed state will be investigated by identifying sites on the mRNAs that become inaccessible when the molecules are not available for translation, using a newly-developed S1 mapping procedure. The project is part of a long range research program aimed at identifying signals that modulate gene expression in mammalian cells. It may also indicate whether the growth characteristics of tumor cells may be determined in part by changes in the extent of repression of individual mRNA species.